The invention concerns impedance matching between an RF power source and a tapped coil source power applicator of a plasma reactor.
Referring to FIG. 1, a plasma reactor for processing a semiconductor workpiece (e.g., a silicon wafer for fabrication of computer chips) includes a reactor chamber 100 with a ceiling 110, the chamber enclosing a wafer support pedestal 115 for holding a semiconductor wafer 120 during processing. Processing gases are introduced into the chamber 100 through gas inlets 125 and are ionized to form a plasma in the chamber 100 by RF power radiated from an overhead coil antenna 130. The coil antenna 130 consists of at least one coiled conductor wound to form a number of windings. RF power is applied to the antenna 130 by an RF plasma source power generator 135 connected at a tap point 137 on an intermediate winding 140 of the coil antenna 135. The circuit is completed by connecting the antenna""s top winding 145 to RF return or ground through a capacitor 150 and connecting the antenna""s bottom winding 155 directly to ground. The capacitor 150 is selected to form a resonant circuit with the inductive coil antenna 130 with a resonance near the desired frequency of the RF generator 135. Generally, the load impedance presented by the combination of the coil antenna 130 and the chamber (both before and after plasma ignition) differs from the output impedance of the RF generator 135. The greater the difference in impedance, the more RF power is reflected back to the RF generator and the less power is delivered to the chamber. For this reason, the typical RF generator itself has a limited capability to maintain the forward power at a nearly constant level even as more RF power is reflected back to the generator as the plasma impedance fluctuates. Typically, this is achieved by the generator servoing its output power level, so that as an impedance mismatch increases (and therefore reflected power increases), the generator increases its output power level. Of course, this capability is limited by the maximum output power of which the generator is capable of producing. Typically, the generator is capable of handling a maximum ratio of forward standing wave voltage to reflected wave voltage (i.e., the voltage standing wave ratio or VSWR) of not more than 3:1. If the difference between impedances increases (e.g., due to plasma impedance fluctuations during processing) so that the VSWR exceeds 3:1, then the RF generator can no longer control the delivered power, and control over the plasma is lost. As a result, the process is likely to fail. Therefore, at least an approximate impedance match must be maintained between the RF generator 135 and the load presented to it by the combination of the coil antenna 130 and the chamber 100. This approximate impedance match must be sufficient to keep the VSWR at the generator output within the 3:1 VSWR limit over the entire anticipated range of plasma impedance fluctuations. The impedance match space is, typically, the range of load impedances for which the match circuit can maintain the VSWR at the generator output at or below 3:1.
One difficulty with the reactor of FIG. 1 is that when RF power is first applied, there is no plasma in the chamber 100. Thereafter, the load impedance undergoes a very large abrupt change upon plasma ignition. This is because after plasma ignition the coil antenna induces mirror currents in the plasma which oppose the coil EMF and thereby effectively reduce the coil inductance. This reduction in inductance changes the load impedance of the coil antenna, so that the pre-plasma ignition load impedance significantly differs from the post-plasma ignition load impedance. The difference between the pre-and post-plasma ignition impedances is so great that it is not possible to provide an optimal impedance match prior to and after plasma ignition. This is because, typically, the impedance match space provided by a conventional fixed impedance match circuit is not sufficiently broad to encompass both the pre-ignition load impedance and the post-ignition load impedance. As stated above, the impedance match space is, typically, the range of load impedances for which the match circuit can maintain the VSWR at the generator output at or below 3:1. Even if the match space were sufficiently broad to encompass both the pre- and post-ignition load impedances, the system would have to be carefully tuned since the margin by which the impedance match space could cover both impedances would be relatively narrow. Thus, the useful impedance match space during plasma processing would necessarily be significantly constricted. As a result, the processing window of the reactor is constricted to avoid swings in plasma load impedance which would take the load impedance outside the constricted impedance match space.
Some compromise must be made in the selection of RF frequency, capacitance of the capacitor 150 and antenna inductance so that the VSWR limitations of the RF generator 135 are met both prior to and after plasma ignition. This situation is illustrated in the Smith chart of FIG. 2, in which reactance is plotted on the imaginary vertical axis and resistance is plotted on the real horizontal axis. Z1 is the pre-plasma ignition load impedance and Z2 is the post-plasma load impedance of an exemplary plasma reactor. Their location is a function of the capacitance of the tuning capacitor 150, which must be carefully selected. With such a selection, the load impedances Z1 and Z2, together with the RF generator output impedance Z0 of 50 Ohms, provide reflection coefficients (Z1/Z0 and Z2/Z0, respectively) that do not exceed the 3:1 VSWR capability of the RF generator 150. However, this condition is satisfied by a small margin, so that the system is susceptible to failure during processing occasioned by wide swings in the plasma impedance.
One compromise that can be made (by an appropriate selection of the tuning capacitor etc. in accordance with conventional techniques) is to center the limited impedance match space around the post-ignition load impedance. This provides an optimum match to the post-ignition load impedance to optimize control during processing. It also provides a correspondingly inferior impedance match to the pre-ignition load impedance which must be, however, sufficient to couple enough power to ignite a plasma. Of course, such an arrangement is unreliable. Alternatively, some type of dynamic impedance matching device must be employed, which would increase system cost and complexity.
Therefore, there is a need to provide a fixed impedance match with a sufficiently large match space to accommodate both the pre-ignition load impedance and the post-ignition load impedance.
The present invention provides a way of following the abrupt impedance change characteristic of plasma ignition without a dynamic impedance matching device.
In a plasma reactor for processing a semiconductor wafer having an overhead inductive coil antenna, automatic compensation for the load impedance shift that accompanies plasma ignition is achieved using fixed elements. This is accomplished by applying RF power to an intermediate tap of the coil antenna that divides the antenna into two portions, while permanently suppressing the inductance of one of the two portions to an at least nearly fixed level. For this purpose, an inductance-suppressing conductive body is held sufficiently close to one of the two portions so as to fix the inductance of the one portion at a suppressed level that is at least nearly constant over plasma ignition, leaving the inductance of the other portion unsuppressed and free to fall when a plasma is ignited and rise when it is extinguished. The resulting change in the ratio of the inductances of the two portions upon plasma ignition automatically compensates for the change in load impedance that occurs upon plasma ignition.